Designs

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waawamellon
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Re: Designs

Post by waawamellon » February 22nd, 2011, 2:01 pm

dragonfly wrote:you'll likely have to spend more time to bend or cut wood into a circle... If your question really comes down to "Should I experiment with towers with circular bases", the answer is that you should not.
i wish i read this a month ago!! D:
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Re: Designs

Post by soccerkid812 » February 22nd, 2011, 2:12 pm

waawamellon wrote:
dragonfly wrote:you'll likely have to spend more time to bend or cut wood into a circle... If your question really comes down to "Should I experiment with towers with circular bases", the answer is that you should not.
i wish i read this a month ago!! D:
why?

i would recommend experimenting with it, but at the same time, be building other ones just in case a curved one is not the best

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Re: Designs

Post by SLM » February 22nd, 2011, 5:01 pm

soccerkid812 wrote:
waawamellon wrote:
dragonfly wrote:you'll likely have to spend more time to bend or cut wood into a circle... If your question really comes down to "Should I experiment with towers with circular bases", the answer is that you should not.
i wish i read this a month ago!! D:
why?

i would recommend experimenting with it, but at the same time, be building other ones just in case a curved one is not the best
I agree. Experimentation is a key factor in designing and building competitive towers (or other engineering devices).

Although we did not experiment with a cylindrical or curved base, we did build a tower with a cylindrical chimney early in the season. Here is a picture of it.
Image

The chimney was very simple to build. We used 1/32"-thick medium density basla for it. We cut the wood to size, soaked it in water for a few hours, then wrapped it around a PVC pipe and used rubber bands to keep it in place until it was dry.

Without the base, the chimney held at least 45 kg (we did not load it past that). But, it weighed about 9 grams. To reduce the weight, we also experimented with 1/64" thickness, but were not able to get it to work satisfactorily.

For us, for a cylindrical chimney, the challenge was to connect the chimney to the base. We even thought of cylindrical or semi-cylindrical legs, but eventually had to abandon the plan due to its complexity and our inability to lower the weight.

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Re: Designs

Post by fanjiatian » February 27th, 2011, 6:27 am

Hi,
If the base is taller than the top,
would a bigger base hold more than a smaller base?

Say, 30 cm tall for the base, and 20 cm tall for the top vs 22cm for the base and 28 cm for the top.


Thanks

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Re: Designs

Post by SLM » February 27th, 2011, 12:23 pm

fanjiatian wrote:Hi,
If the base is taller than the top,
would a bigger base hold more than a smaller base?

Say, 30 cm tall for the base, and 20 cm tall for the top vs 22 cm for the base and 28 cm for the top.


Thanks
Let's say we have two towers (A and B). The base for tower A has a height of 30 cm, and tower B has a base with a height of 22 cm. If both towers have the same general shape, say a rectangular base and a square chimney, then A has longer legs than B. Let's say the legs for A are about 31 cm each and the legs for B are 23 cm each.

If all the legs have the same cross-sectional dimensions (say 1/8" x 1/8"), and they are all properly braced against buckling (say every 4 cm in either direction using X-bracing), then the legs of A and B have the same load-carrying capacity. Let's refer to this capacity as Pmax. When the force in a leg, in either tower, reaches Pmax, the leg fails.

So, the question is in which tower the force in a leg reaches Pmax first?

Since A has a taller base, then its legs are less inclined relative to the vertical axis compared to B. Say, the angle between the vertical axis and a leg for A is 15 degrees. Then, the same angle in B would be about 20 degrees. The bigger the inclination angle the more force is distributed to the leg. This means, a leg in B is subjected to more force(in this case by about 3% more) than a leg in A. Therefore, if the force applied to the tower is large enough, the force in a leg in B reaches Pmax first; B would fail before A does.

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Re: Designs

Post by deezee » February 27th, 2011, 2:08 pm

thanks, I was wondering that too :D
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Re: Designs

Post by 12thomasal » February 27th, 2011, 11:36 pm

SLM, you do realize that your circular tower would medal in many states

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Re: Designs

Post by hogger » February 28th, 2011, 8:20 am

Drill some regularly spaced holes on the cylindrical sheet. I think you can get a few grams out of it easily especially if it holds 45 kg.

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Re: Designs

Post by SLM » February 28th, 2011, 10:44 am

hogger wrote:Drill some regularly spaced holes on the cylindrical sheet. I think you can get a few grams out of it easily especially if it holds 45 kg.
First, a clarification: the weight of the cylindrical chimney, not the entire tower, is 9g. The tower, I believe, weighs about 13g.

It is true that drilling holes in the chimney brings its weight down. But, three main issues need to be considered when trying to reduce the weight by taking material away:

1. Holes probably cannot be too big. I would say they should not exceed 2cm in diameter or else the cylindrical member losses its excellent resistance against buckling.

2. Unless the holes are very small, they need to be aligned vertically (not staggered) to allow the applied force to be transferred to the base of the cylinder through the vertical wood grains. If the force being transmitted down the chimney has to go around a hole, especially if the hole has a big diameter , then the wood is going to be stressed in its weaker direction, something that needs to be avoided.

3. Given the thickness of the wood (1/32"), care has to be exercised when drilling or punching holes in order to prevent damaging the wood. A hole puncher generally doesn't work unless you can find one with a long neck; drilling is probably a better option. We experimented with a hole puncher that creates 2 mm diameter holes. It took forever to punch sufficient number of holes to reduce the weight by 1/3. It was time consuming and required precision and patience in punching all the holes.

This year, we don't have time to run more experiments on cylindrical chimneys. But, if someone is interested in pursuing the idea further and try to make it work, I would be more than happy to help out any way I can.
Last edited by SLM on February 28th, 2011, 6:40 pm, edited 1 time in total.

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Re: Designs

Post by Frogger4907 » February 28th, 2011, 10:55 am

you could drill out slots out of the middle of the strips with a cnc machine and some small sized end mills, this would greatly help reducing weight.
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